EP4174587A1 - Vorrichtung zur verarbeitung von zeitstempeln - Google Patents

Vorrichtung zur verarbeitung von zeitstempeln Download PDF

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Publication number
EP4174587A1
EP4174587A1 EP21205642.8A EP21205642A EP4174587A1 EP 4174587 A1 EP4174587 A1 EP 4174587A1 EP 21205642 A EP21205642 A EP 21205642A EP 4174587 A1 EP4174587 A1 EP 4174587A1
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EP
European Patent Office
Prior art keywords
timestamps
time
periodic
timestamp
computer program
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Pending
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EP21205642.8A
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English (en)
French (fr)
Inventor
Markus Wick
Helmut Fedder
Michael SCHLAGMÜLLER
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Swabian Instruments GmbH
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Swabian Instruments GmbH
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Priority to EP21205642.8A priority Critical patent/EP4174587A1/de
Priority to PCT/EP2022/078844 priority patent/WO2023072663A1/en
Publication of EP4174587A1 publication Critical patent/EP4174587A1/de
Pending legal-status Critical Current

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    • G—PHYSICS
    • G04—HOROLOGY
    • G04F—TIME-INTERVAL MEASURING
    • G04F10/00—Apparatus for measuring unknown time intervals by electric means
    • G04F10/005—Time-to-digital converters [TDC]

Definitions

  • the invention concerns an apparatus for processing timestamps generated by two or more time-to-digital converters (TDCs) that are driven by a common reference clock.
  • TDCs time-to-digital converters
  • the described apparatus improves the accuracy of timestamps generated from periodic or quasi periodic signals and enable switching the time reference of timestamps.
  • the described apparatus is particularly useful for test and measurement applications where precise timestamps are compared, such as fluorescence lifetime measurements, clock comparison measurements, photonic quantum computing and quantum communication.
  • a time tagger is an instrument that is used to detect edges of an electronic signal and to determine their time of arrival. It typically comprises two or more TDCs that are driven by a common reference clock. For each edge of an input signal, each TDC outputs a timestamp that encodes the time-difference between the input signal edge and an edge of the reference clock signal.
  • An electronic circuit or computer program merges and sorts the timestamps from all TDCs into a data stream with increasing order according to their occurrence in time and combines each time value with a number that identifies the TDC from which it originated.
  • the stream generator also counts the edges of the reference clock and converts the time values into timestamps relative to a specific clock edge that corresponds to time zero, meaning that the timestamps in the data stream are now values larger than the period of the reference clock.
  • the stream generator may also include counter rollover markers in the data stream. This data stream of timestamps is received by a digital electronic circuit or computer program and further processed in a way that is specific to each measurement application.
  • one TDC receives the signal from a single photon detector and a second TDC receives the signal from a pulsed laser. For each photon timestamp, the time difference to the preceding or subsequent laser timestamp is calculated and counted in a histogram. In this way, a fluorescence decay curve of a sample is obtained.
  • two independent clock oscillators that output one-pulse-per-second (1 PPS) signals and 10 MHz clock signals are connected to a time tagger.
  • the time difference between the two 1 PPS pulses is calculated and logged into a continuous plot that represents the relative timing accuracy and clock drift of the GNSS receivers over time.
  • the 10 MHz clock signals are compared to each other and an Alan deviation is calculated from the timestamps of the clock edges of both clocks, providing a quantitative analysis of the long term clock stability, including long term drift, aging, phase noise spectrum, etc.
  • a clock dividers are used to first divide both 10 MHz signals to slower signals, for example 1 MHz, in order to reduce the data rate of the measured timestamps.
  • the TDCs are used to realize a multi-channel logic analyzer.
  • the timestamps of all rising and falling edges are streamed to a computer and from the timestamps a plot is created that visualizes the original digital signals.
  • the TDC receives the signals from an array of single photon detectors and a computer program processes the timestamp data stream to identify timestamps that coincide with timestamps on one or more other channels within a certain time window.
  • Yet another application is a LIDAR or laser ranging measurement, where the trigger signal of a pulsed light source is connected to one TDC and an optical detector is connected to a second TDC.
  • a digital circuit or computer program calculates the time difference between the timestamps from the trigger from the light source and the optical detector, which is the time of flight, and converts this into a distance.
  • time tagger offers a generic and versatile solution to these diverse measurement tasks.
  • one of the signals that is detected and processed by the time tagger is a periodic signal.
  • a time tagger has specific disadvantages that we describe below.
  • Fig. 1 uses a single TDC.
  • the periodic pulsed laser is applied to the "stop” input of the TDC and the single photon detection events are applied to the "start” input of the same TDC.
  • single photon events are conceptually “stop” events (photons are fluorescence light that is emitted after a laser excitation)
  • this scheme is sometimes called a "reverse-start-stop-measurement” in the literature [B&H single photon counting book].
  • This scheme performs a direct measurement of the time-difference between a single photon detection event and a subsequent laser trigger. These time-differences can then directly be accumulated in a histogram.
  • TDCs suffer from electronic jitter that results in an inaccuracy of the measured time-difference between the start and stop signals.
  • the measurement jitter occurs once.
  • the measurement jitter occurs twice. Consequently, for typical gaussian distributed jitter, the total jitter of a measured time difference is worse by a factor SQRT(2).
  • the pulsed laser has an excellent clock accuracy in terms of absolute frequency - for example when a mode locked laser is used that has a well calibrated resonator length.
  • the period of the pulsed laser serves as the reference time for the measurement.
  • the absolute measure of time is indeed given by the repetition rate of the stable pulsed laser, whereas, when using a time tagger, the absolute measure of time is determined by the reference clock oscillator, which is often less accurate as compared to the laser.
  • DE102008004549B4 "Apparatus and method for simultaneous time resolved single photon registration from a plurality of detection channels" describes a time tagger as detailed above, characterized by the aspect that each TDC generates a rollover marker, and characterized further by the fact that the apparatus is applied in the context of multi channel single photon counting, and characterized further by its specific timing resolution between 0.5 and 50 ps.
  • US10715754B2 Single reference clock time to digital converter discloses a method related to LIDAR and similar time-of-flight measurements with SPAD arrays that uses a multi-channel TDC with a common reference clock and processing logic for the signals from a SPAD array, where the common reference clock of the TDCs is synchronized with the laser pulse generator. While in that patent DSP signal processing of time-differences is described to evaluate measured time-differences and compute distances, the rescaling and refining data processing as described in the present submission, was not disclosed.
  • the description provides an apparatus that improves the accuracy of timestamps related to periodic or quasi periodic signals and enables rescaling of the time basis of timestamps.
  • the description employs processing methods that operate on timestamp data streams with low latency, for example less than a second, thereby generating new timestamp data streams.
  • the description employs refinement calculations on a timestamp data stream.
  • a highly robust averaging calculation can effectively be realized by a control loop mechanism that continuously processes the timestamp data stream.
  • the jitter of a timestamp can be improved by a factor SQRT(N) by taking into account N earlier or later timestamps that occurred at known multiples of a clock period.
  • SQRT(N) the jitter of a timestamp can be improved by a factor SQRT(N) by taking into account N earlier or later timestamps that occurred at known multiples of a clock period.
  • timestamps in a data stream that originate from a specific TDC are replaced by more accurate timestamps. These can then be used for further processing.
  • the invention also applies rescaling of timestamps in a timestamp data stream, where a periodic input signal that is applied to one TDC is considered to be a reference clock signal and a suitable algorithm such as linear interpolation rescales the timestamps that originate from other TDCs.
  • a rescaling calculation is useful, as it allows to express the values of measured timestamps relative to an external clock signal that is distinct from the internal reference clock and distinct from a special provided external hardware reference clock. Instead timestamps can now be expressed with respect to time information that is contained in any other input signal. This enables a flexible way for the user to switch between time references.
  • the averaging and rescaling calculations are performed on-the-fly on the data stream, meaning with a low latency, typically below one second. This ensures that the data can immediately be further processed or displayed. Being able to see measurement results in real time is an important added value for many applications.
  • Such processing of the timestamp data stream may be performed in a dedicated electronic circuit, such as an FPGA core, or it may be performed by a suitable software program that is executed by a processing unit.
  • FIG. 6 A possible embodiment of the invention is illustrated in Fig. 6 .
  • the embodiment is used for example to realize a fluorescence lifetime measurement with lower jitter and with an improved absolute time reference by exploiting both continuous refinement calculations of the timestamps of a periodic signal and by exploiting continuous interpolation calculations of timestamps to achieve a change of the time reference frame.
  • a periodic pulsed laser with a repetition rate of 80 MHz generates a periodic signal (P) that is connected to a first input (CH#A) of a time tagger (TT).
  • a single photon detector generates a trigger signal (S) for each single photon detection event and is connected to a second input (CH#B) of the time tagger.
  • a quartz and a PLL generate a stable 500 MHz reference clock signal (CLK).
  • TDCs (TDC#A, TDC#B) are implemented in a Field-Programmable-Gate-Array using tapped delay lines.
  • the 500 MHz reference clock is applied to the start inputs of the two TDCs.
  • the start inputs are embodied by the latches of the tapped delay lines.
  • the 80 MHz periodic trigger signal and the single photon detector signal are applied to the two stop inputs of the TDCs.
  • the stop inputs are embodied by the delay line inputs.
  • Decoders convert the tapped-delay-line outputs into time differences that provide time values that are at maximum the period of the reference clock, i.e. 2 ns.
  • a stream generator sorts the time values from the TDCs and combines each time value with a number that identifies the TDC from which it originated.
  • the stream generator also includes a counter that counts the edges of the reference clock and converts the time values into timestamps relative to a specific clock edge that corresponds to time zero, meaning that the timestamps in the data stream are now values much larger than the 2 ns clock period.
  • the timestamps are expressed by integers and an integer number of one corresponds to one picosecond, whereby the 500 MHz reference clock is assumed to have a period of precisely 2 ns, i.e. the 500 MHz reference clock is the absolute time reference.
  • the stream generator also includes counter rollover markers in the data stream.
  • This timestamp data stream (TD#1) is continuously streamed to a PC, where it is received by a software that includes a receiving engine.
  • the receiving engine converts the time values into 64 bit unsigned integers, taking into account the rollover markers.
  • the timestamp data stream now represents monotonously increasing time values that rollover only after about 0.3 years. So, for a typical application, no rollover is expected to happen and for the sake of all further processing the timestamp data stream can be considered as a stream of consecutive monotonically increasing absolute timestamps.
  • the timestamp data stream is processed by a refining engine (E-REF) that continuously modifies the timestamps of the periodic laser trigger thereby providing and outputting a modified timestamp data stream (TD#2).
  • the refining engine employs a PID controller that holds and continually refines a value for the period T n .
  • proportional, integral and differential parameters are used along with the error, its discrete derivative and its integral to calculate an updated period T n .
  • the output timestamp data stream corresponds to the input timestamp data stream except that for all laser timestamps, the original time value is replaced by the refined time value t' P,n .
  • Careful selection of the PID parameters is beneficial to achieve robust and accurate operation.
  • the PID parameters are expressed in terms of a damping parameter, which is typically tuned according to the aperiodic limit case, a low pass filter parameter, which is typically selected to be 10 periods, and a periodicity parameter, which is typically selected to be the period.
  • the final fluorescence lifetime data is obtained by a data processing engine that consumes timestamp data stream TD#1 and calculates for each photon timestamp the time difference to the subsequent laser timestamp and accumulates these time differences in a histogram. Thereby a fluorescence lifetime curve is obtained where the electronic jitter from TDC#A has essentially been removed. The only jitter that remains is the jitter of the single photon detector and electronic jitter of the TDC that processes the triggers from the single photon detector (TDC#B) .
  • the embodiment is directed to compare two clocks. Specifically, quantities such as Allan deviation and Maximum Time Interval Error (MTIE) of one clock are calculated using the other clock as an independent reference.
  • a reference clock for example an atomic clock outputs a periodic 10 MHz clock signal (P) that is connected to a first input (CH#A) of a time tagger (TT).
  • the frequency of the atomic clock is assigned to be nominally exactly 10 MHz, i.e., its period is assigned to be nominally exactly 100 ns.
  • a second clock outputs a second 10 MHz signal that is applied to a second input (CH#B) of the time tagger.
  • a first step refinement calculations of the timestamps corresponding to the atomic clock are performed as described for the case of the fluorescence lifetime measurement, generating a second timestamp data stream (TD#2) containing refined timestamps t' P,i .
  • a continuously executed rescaling calculation linearly interpolates the timestamps of the second channel t S,i relative to the refined timestamps, thereby generating new timestamps t* S,i that express the timestamps of the second channel in the time reference frame of the atomic clock.
  • a control loop mechanism calculates a refined timestamp t' P,i as described above.
  • the refined timestamps are now used to linearly interpolate the timestamps t S,i of the second clock, expressing them in the time frame of the atomic clock.
  • the relation of the initial, the refined and the rescaled timestamps is illustrated in Fig. 7 .
  • the n'th timestamp of the second clock appears between the n'th and (n+1)'th timestamp of the refined atomic clock.
  • TD#3 a new timestamp data stream
  • the timestamps of the second clock are replaced by such linearly interpolated timestamps

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Unknown Time Intervals (AREA)
  • Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)
EP21205642.8A 2021-10-29 2021-10-29 Vorrichtung zur verarbeitung von zeitstempeln Pending EP4174587A1 (de)

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Application Number Priority Date Filing Date Title
EP21205642.8A EP4174587A1 (de) 2021-10-29 2021-10-29 Vorrichtung zur verarbeitung von zeitstempeln
PCT/EP2022/078844 WO2023072663A1 (en) 2021-10-29 2022-10-17 Apparatus for timestamp processing

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EP21205642.8A EP4174587A1 (de) 2021-10-29 2021-10-29 Vorrichtung zur verarbeitung von zeitstempeln

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030076181A1 (en) * 2000-03-17 2003-04-24 Sassan Tabatabaei Tunable oscillators and signal generation methods
US20090074124A1 (en) * 2007-09-16 2009-03-19 Infineon Technologies Ag Determining a Time Interval Based on a First Signal, a Second Signal, and a Jitter of the First Signal
US20090072812A1 (en) 2007-09-14 2009-03-19 Infineon Technologies Ag Event-Driven Time-Interval Measurement
WO2012137109A2 (en) * 2011-04-05 2012-10-11 Koninklijke Philips Electronics N.V. Detector array with time-to-digital conversion having improved temporal accuracy
DE102008004549B4 (de) 2008-01-15 2013-04-18 PicoQuant GmbH. Unternehmen für optoelektronische Forschung und Entwicklung Vorrichtung und Verfahren zur simultanen zeitaufgelösten Einzelphotonenregistrierung aus einer Mehrzahl von Detektionskanälen
US10715754B2 (en) 2018-01-23 2020-07-14 Stmicroelectronics (Research & Development) Limited Single reference clock time to digital converter

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030076181A1 (en) * 2000-03-17 2003-04-24 Sassan Tabatabaei Tunable oscillators and signal generation methods
US20090072812A1 (en) 2007-09-14 2009-03-19 Infineon Technologies Ag Event-Driven Time-Interval Measurement
US7804290B2 (en) 2007-09-14 2010-09-28 Infineon Technologies, Ag Event-driven time-interval measurement
DE102008046831B4 (de) 2007-09-14 2017-12-07 Intel Deutschland Gmbh Ereignisgesteuerte Zeitintervallmessung
US20090074124A1 (en) * 2007-09-16 2009-03-19 Infineon Technologies Ag Determining a Time Interval Based on a First Signal, a Second Signal, and a Jitter of the First Signal
DE102008004549B4 (de) 2008-01-15 2013-04-18 PicoQuant GmbH. Unternehmen für optoelektronische Forschung und Entwicklung Vorrichtung und Verfahren zur simultanen zeitaufgelösten Einzelphotonenregistrierung aus einer Mehrzahl von Detektionskanälen
WO2012137109A2 (en) * 2011-04-05 2012-10-11 Koninklijke Philips Electronics N.V. Detector array with time-to-digital conversion having improved temporal accuracy
US10715754B2 (en) 2018-01-23 2020-07-14 Stmicroelectronics (Research & Development) Limited Single reference clock time to digital converter

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